JPH068984B2 - Projection type full color liquid crystal display - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a projection type liquid crystal display device for full color display, and more particularly to a structure of a TN type liquid crystal cell which constitutes a part of an optical system thereof.

2. Description of the Related Art Conventionally, a projection-type full-color display device using a TN liquid crystal panel instead of the CRT has been proposed in order to reduce the size and weight of the system, as compared with a projection-type full-color display device using a CRT. . An outline of a projection type full color display device using this TN liquid crystal cell will be described below with reference to FIG.

In FIG. 7, the white light emitted from the light source 1 is converted into R light (6
10 nm), G light (550 nm), B light (450 nm)
The light is divided into the wavelength bands of R, and the respective lights are sandwiched by the polarizing plates 8 and 8'arranged in front and rear in a substantially parallel Nicol state.
The light is incident on three TN liquid crystal cells for G, G, and B, and the image formed on the liquid crystal cells is focused on the screen 10 through the condensing optical system 9.

When the three color lights of RGB are transmitted to three TN liquid crystal cells and the images formed on the TN liquid crystal cells are formed on the same screen, the emitted light depends on the wavelength of the incident light to the TN liquid crystal cells. Of these RGB due to the different polarization states of
It is necessary to separately use a TN liquid crystal cell whose optical design is optimized. Therefore, as a method generally used, Go
och-Tarry's formula (T: transmittance, Δn: refractive index anisotropy, d: cell gap, λ: wavelength), each liquid crystal cell has a value of the cell gap d at which the transmittance T is minimized at each wavelength of R, G and B. To form.

That is, the first peak at which the transmittance value by the Gooch-Tarry equation becomes the minimum The gap thicknesses are set as shown in the table below so that the wavelengths of R, G, and B colors coincide with each other, and TN liquid crystal cells for Blue, Green, and Red may be manufactured.

On the other hand, according to this calculation, three panels were manufactured, and R,
When the contrast of the G and B liquid crystal cells was measured by a luminance meter by the voltage-transmittance characteristic, the results shown in the table below were obtained.

As can be seen from the above results, a cell having a gap thickness that theoretically provides the optimum contrast should be realized, but the contrast ratio of the cell for blue is not sufficiently obtained. The possible causes are as follows.

(1) The spectral characteristic of the light source incident on the B cell is broad.

(2) Optical rotation tends to occur in the blue wavelength range.

(3) The birefringence of liquid crystal becomes small in the low wavelength region.

The above causes will be described in detail with reference to FIGS. 6A, 6B, 6C and 6D.

FIG. 6a shows 90 ° TN according to the Gooch-Tarry equation described above.
The transmittance of the liquid crystal is plotted against the wavelength. The curves indicated by the two-dot chain line 41, the solid line 42, and the one-dot chain line 43 are for blue (gap thickness d = 4.2 μm).
m), for Green (d = 5.0 μm), for Red (d = 5.7 μm)
It corresponds to the TN liquid crystal cell of. As can be seen from this figure, the optical properties of liquid crystals are generally in the short wavelength region (Blue
Side), the band that can block light is considerably narrower than the long wavelength region (Red side). On the other hand, as shown in FIG. 6b, light incident on each TN liquid crystal cell, that is, blue light (44
Is the Green light (45 solid line), Red light (46
It is a broad curve with a wider band than the one-dot chain line. Therefore, as a result of multiplying the characteristics of FIG.
Most of the een light and the red light are blocked, but with the blue light, the light slightly deviated from the peak value cannot be blocked, and light leakage due to optical rotatory dispersion is likely to occur.

Here, in order to prevent this, the characteristic of the Blue light source 44 may be sharpened. However, as shown in the figure of the sensitivity of the human eye to the wavelength in FIG.
The sensitivity of the human eye is low in the area of ue light. Because of this, Blue
In order to obtain sufficient brightness of light, the characteristics of the blue light source must be as broad as possible, and these are in a mutually contradictory relationship.

Further, as shown in FIG. 6d, the birefringence value in the wavelength range of the blue light is also in terms of the wavelength dependence of the birefringence of the liquid crystal.
It is not constant like the wavelength range of Green light and Red light, and the unique gap value by the Gooch-Tarry theoretical formula is not effective in the whole of the short wavelength range, and this is also due to the low contrast of the cell for Blue. is there.

Means for Solving the Problems In order to solve the above problems, the present invention proposes the following configuration of a projection type full color liquid crystal display device. That is, (1) a structure in which a TN type liquid crystal cell having the same cell gap as that of Blue and having the same twist angle of liquid crystal molecules but the opposite twist direction is superposed on the liquid crystal cell for blue.

(2) Red, Green, and Blue TN liquid crystal cells and Blu
All of the TN type liquid crystal cells to be superposed on the e-type cell have the same cell gap, these display formats are normally black, and the front and rear polarizers of the red and green TN type liquid crystal cells are detected. Optimizing the transmittance characteristics for each R, G, B by setting the direction of the polarization axis of the photon at an appropriate angle with respect to the alignment direction of the liquid crystal molecules on the light incident side and the light emitting side of the TN type liquid crystal cell. To do.

(3) Further, as the angle setting of the polarization axis described above, the case where the polarization axis is shifted in the direction in which the optical rotation angle of the linearly polarized light passing through the TN type liquid crystal cell decreases is positive, and the polarization axis before and after the liquid crystal cell for Red light is The intersection angle is set to -10 ° to 0 °, and the polarization axis intersection angle before and after the liquid crystal cell for Green is set to 0 ° to + 10 °.

Effect According to the present invention, even for a liquid crystal cell for blue light, which is difficult to obtain a high contrast if it is a single cell, its rotatory dispersion is compensated by the superposed TN liquid crystal cell.
There is no leakage of light that is slightly off the peak of Blue light,
It is possible to realize a projection type full-color liquid crystal display device having high contrast in R, G, and B and good color reproducibility.

Further, as described in the above item 2, it is advantageous in manufacturing the TN liquid crystal cell if all the cells including the liquid crystal cell for optical compensation have the same gap. That is, a large number of liquid crystal cells having the same specifications can be manufactured and a system can be configured using liquid crystal cells having uniform performance. Since the cells have the same gap, the response characteristics to the applied voltage of the liquid crystal are the same,
Almost the same voltage-transmittance characteristics can be realized for R, G, and B.

Embodiments Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an example of an outline of an optical system in a projection type full-color liquid crystal display device according to the present invention, and the same reference numerals are used for the same components as in FIG. 7 described as a conventional example in the drawings.

As shown in FIG. 1, an optical compensation TN liquid crystal cell 7'for compensating the optical rotatory dispersion is superposed on the Blue TN liquid crystal cell. Here, 1 is a white light source 2, 3 and 4 are dichroic mirrors having a reflection spectral characteristic having peaks at wavelengths corresponding to R, G and B, respectively, whereby white light from the light source is converted into R, G and B color lights. Will be divided. Reference numeral 8 is a polarizer for converting the light incident from the dichroic mirror into linearly polarized light, and 8'is an analyzer installed so that the oscillation direction of the linearly polarized light emitted from the cell with no voltage applied to the cell coincides with the transmission axis. That is, they are in the form of normally black. 5, 6 and 7 are for R and G respectively
, B TN liquid crystal cell, each gap is the above-mentioned Gooch-Tarry minimum transmittance condition The cell gap is set to satisfy 9
Is an optical system including a lens group that magnifies and projects the image on the cell on the screen 10.

Here, the optical characteristics of the blue cell will be described with reference to FIG.

The blue light reflected from the dichroic mirror having a spectral reflectance peak in the blue wavelength band is light having an oscillating direction in all directions as shown by 20, and this is the polarizer 11
As a result, linearly polarized light having a vibration direction as shown by 15 is made incident on the cell 12. The light emitted from this cell 12 swirls so as to follow the twist of the liquid crystal molecules,
In the case of N liquid crystal, it should come out as linearly polarized light that is almost orthogonal to the incident linearly polarized light, but as described above, the optical property of liquid crystal is that it can block light in the normally black mode in the short wavelength region. The wavelength range is narrow, whereas Bl
When blue light having a relatively wide band enters to increase the brightness of ue light, elliptically polarized light as shown in 16 is emitted. Therefore, when cells 13 having the same cell gap as 12 and having the same twist angle of liquid crystal molecules but opposite directions are overlapped, the cells which follow the reverse process of the change of the polarization state of the light passing through exactly 12 cells are Since light emerges at the exit surface of 13, the light becomes a linearly polarized light having the same vibration direction as the linearly polarized light incident on the cell 12. Therefore, the blue light is almost completely blocked by the analyzer 14 having the absorption axis 18 parallel to the linearly polarized light 17. Also, with this method, the birefringence of the liquid crystal
It can also handle changes in the blue light range.

On the contrary, when transmitting light, the same voltage may be applied to the cells 12 and 13, and the light loss due to such a two-layer structure is about 20% or less. The value of the contrast is significantly improved because the value of the denominator T _{v = 0} in is sufficiently small.

Furthermore, FIG. 3 shows an outline of the configuration of an optical system of a projection type full color liquid crystal display device in another embodiment of the present invention.

In FIG. 3, description of the same parts as those in FIG. 1 will be omitted. The projection type full-color liquid crystal display device shown in FIG. 3 is characterized in that all TN liquid crystal cells have the same cell gap, including an optical compensation liquid crystal cell that is superposed on blue light. Here, R, G, and B cells 5a, 6a,
Alignment directions of liquid crystal molecules 7a and 7a ', and polarizers 8a, 8b, 8c and an analyzer 8 arranged before and after the cell.
The relationship between the polarization axes of a ', 8b' and 8c 'will be described with reference to FIGS.

Fig. 5 is a plot of the gap value that minimizes the transmittance of the TN liquid crystal cell when the angle of the polarization axis of the polarizing plate is changed. This is shown by Goscianski in J.appl.Phys.48.
(4), p1427 (1977), a conditional expression theoretically derived as a condition that incident linearly polarized light passes through a 90 ° TN cell and is output as linearly polarized light. It is due to.

Here, the parameter u is u = 2Δnd / λ (Δn: birefringence of liquid crystal, d: cell gap, λ: wavelength), and α represents the angle of the polarization axis with respect to the alignment direction of the light incident surface of the liquid crystal cell. When the polarization axis of the incident side is shifted in the direction of rotation of the linearly polarized light of the TN cell with respect to the alignment direction of the incident side, and the polarization axis is shifted in the direction of decreasing the optical rotation angle of the linearly polarized light at the emission side, the angle is positive. It is defined that the condition that the emitted light is linearly polarized is that the angle of intersection between the incident-side polarization axis and the emission-side polarization axis is 2
This is the case of α.

FIG. 5 shows Δn in the above Goscianski conditional expression.
= 0.0935, λ ₁ = 450 nm (dotted line 36), λ
₂ = 545 nm (solid line 35), λ ₃ = 610nm respectively 90 ° TN cell for (dashed line 34) represents the relationship between the d-alpha. As you can see from Figure 5, for example Green
A cell having a cell gap (5.0 μm) is prepared, and α _B =, where the direction in which the optical rotation angle of the linearly polarized light in the cell decreases is positive with respect to the orientation direction of the liquid crystal molecules on the light incident surface of the cell.
Shift the polarization axis on the incident side by + 6.6 °, α _R = -4.0 °,
The crossing angle with the polarization axis on the output side is 2α _B = + 13.2 °, 2α _R =
With −8.0, a cell having a minimum transmittance peak at 450 nm and a minimum transmittance peak at 610 nm can be realized.

Regarding Blue, as described above, even if the gap is optimized or the polarization axis crossing angle is optimized, Blue
If the characteristics of the light source are broad, the contrast value does not rise sufficiently.

On the other hand, in the configuration of the present invention, regarding the blue, the optical characteristics of the blue cell are optimized by superimposing the TN cell having the reverse twist structure and the same cell gap on the cell for blue. For example, even if the cell gap is considerably deviated from the original cell gap for Blue, it is sufficient that the gap with the reverse-twisted TN cell to be overlapped is the same. Therefore, the above-mentioned optimization method of the polarizing plate intersection angle is
It may be applied only to the een cell and the red cell.

FIG. 4 is a diagram showing the relationship between the orientation directions of the R, G, and B cells and the polarization axis intersection angle in the example of the present invention.

FIG. 4a is a diagram showing a 90 ° TN liquid crystal cell for Blue in the constitution of the present invention (in FIG. 4, all the light goes to the front side from the back of the paper). It is parallel to the alignment direction 21 on the incident surface of the cell. The incident linearly polarized light 25 having various vibration directions rotates about 90 ° in the first layer of the cell, and the main vibration direction 2 is substantially parallel to the orientation direction 22 on the exit surface of the first layer.
It is an elliptically polarized light with 6 incident on the cell of the second layer which is further oriented in the direction of 23, and follows the process opposite to that of the first layer and is parallel to the orientation direction 24 on the exit surface of the second layer. It becomes a linearly polarized light 27 and exits from the cell of the second layer.
Therefore, the direction 28 orthogonal to the vibration direction of this linearly polarized light 27
If the analyzer is placed with the transmission axis aligned with, there is almost no light leakage when no voltage is applied.

Next, referring to FIGS. 4b and 4c, Green in the configuration of the present invention
The red cell and the red cell will be described. The cells for R, G, and B shown in FIG. 4 are all made of the same cell gap, including the cells to be overlapped with the cells for Blue, and the gap is set to a value between 5.0 μm and 5.7 μm. There is.

In FIG. 4b, the above-mentioned values are obtained according to the cell gap with respect to the alignment direction 52 on the light incident surface of the green cell.
When linearly polarized light having an oscillation direction 53 deviated by an angle α _G = 0 to 4.3 ° obtained from the Goscianski conditional expression is incident, it has a vibration direction 54 deviated by an angle α _G from the orientation direction 52 on the exit surface. It appears as linearly polarized light, and the transmission axis of the analyzer may be set in the direction 55 orthogonal to this. Therefore, as the polarizing plate crossing angle in this case, the direction in which the optical rotation angle of the linearly polarized light incident on the cell decreases is defined as 2α _G
It may be set to = 0 to + 8.6 °.

On the other hand, consider a Red cell as shown in FIG. 4c. Here, the orientation direction 61 on the light incident surface of the cell is
Against the aforementioned Goscians according to that gap as well
If linearly polarized light having an oscillation direction 63 deviated by an angle α _R = -4.2 ° to 0 ° obtained from the conditional expression of ki is incident, an oscillation direction deviated by an angle α _R from the orientation direction 61 on the exit surface. Since the linearly polarized light 64 having the value of ## EQU1 ## is emitted, the transmission axis of the analyzer may be set in the direction 65 orthogonal to this.
In this case, the polarizing plate crossing angle is 2α according to the definition of the above-mentioned symbols.
_R = -8.4 ° to 0 °.

Therefore, including the cell gap deviation during panel manufacturing,
If the polarization axis is shifted in the direction in which the optical rotation angle of the linearly polarized light passing through the TN type liquid crystal cell decreases, the crossing angle of the polarization axes before and after the liquid crystal cell for Red light is set to -1 according to the cell gap.
0 ° to 0 °, the polarization axis crossing angle before and after the green liquid crystal cell is 0
It may be set to each of ° to + 10 °.

In the structure of the embodiment of the present invention, all the R, G and B cells need to be displaced by a very small angle as compared with the case where the polarization axis crossing angles before and after the R, G and B cells are produced, which is also a major feature of the present invention. one of.

EFFECTS OF THE INVENTION As described above, according to the present invention, it is possible to improve all the contrasts for each RGB with a relatively simple configuration.
Therefore, it is possible to realize a projection type full-color liquid crystal display device having high color reproducibility and good image quality, and it becomes advantageous in manufacturing cells by making all RGB cells cells with the same gap, which is of great industrial value. It is a thing.

[Brief description of drawings]

FIG. 1 is a block diagram showing the outline of an optical system of a projection type full color liquid crystal display device in an embodiment of the present invention, and FIG.
FIG. 3 is a perspective view showing the relationship between a cell for blue used in the optical system shown in the figure and a liquid crystal cell for optical compensation superposed on it, and FIG. 3 shows a projection type full color liquid crystal display device in another embodiment of the present invention. It is a figure showing an outline of an optical system,
FIGS. A, b and c are configuration diagrams showing the relationship between the alignment direction and the polarization axis of the R, G and B TN liquid crystal cells used in the embodiment shown in FIG. 3, and FIG. 5 is a TN based on the Gosianski theory.
A diagram showing the relationship between the cell gap of the liquid crystal cell and the angles of the polarization axes before and after it, FIG. 6a is a transmission spectral characteristic diagram of the TN liquid crystal cell based on the Gooch-Tarry theory, and FIG. 6b is an embodiment of the present invention. FIG. 6c is a diagram showing the spectral response of the dichroic mirror used in the apparatus of the example, FIG. 6c is a diagram showing the sensitivity curve of the human eye, FIG. 6d is a diagram showing the wavelength dependence of the birefringence of the liquid crystal, and FIG. It is a block diagram which shows the outline of the optical system in the conventional projection type full color liquid crystal display device. 1 ... White light source, 2, 3, 4 ... Dichroic mirror, 5 ... Red TN liquid crystal cell, 6 ... Green TN liquid crystal cell, 7 ... Blue TN liquid crystal cell, 7 '... Blue optics Compensation TN liquid crystal cell, 8 ... Polarizing plate, 8 '... Analyzer plate, 9
...... Condensing optical system, 10 …… Screen.

Claims (3)

[Claims] 1. R, G, and B lights of different colors are separately TN.
It is a projection type full-color liquid crystal display device having an optical system for making an image to enter into a liquid crystal cell, color-synthesizing the images on each liquid crystal cell, and enlarging and projecting it on the same screen. A projection type full-color liquid crystal display device having a structure in which TN type liquid crystal cells having the same cell gap and the same twist angle of liquid crystal molecules but opposite twist directions are stacked. 2. The red, green, and blue TN type liquid crystal cells and the TN type liquid crystal cells to be superposed on the blue cell all have the same cell gap, and these display formats are normally black. The directions of the polarization axes of the polarizer and the analyzer before and after the TN liquid crystal cell for Red and Green are TN respectively.
By setting an appropriate angle with respect to the alignment direction of the liquid crystal molecules on the light incident side and the light emitting side of the type liquid crystal cell, R, G,
The projection type full-color liquid crystal display device according to claim 1, wherein the transmittance characteristic for each B is optimized. 3. A case where the polarization axis is shifted in the direction in which the polarization angle of linearly polarized light passing through a TN type liquid crystal cell decreases is defined as positive.
The crossing angle of the polarization axes before and after the liquid crystal cell for d light is -10 ° to 0.
°, the polarization axis crossing angle before and after the liquid crystal cell for Green is 0 ° to +1
The projection type full color liquid crystal display device according to claim 2, wherein the projection type full color liquid crystal display device is set to 0 °.